What is this weird thing when i'm trying to merge the head with the body on a sculpt and how do i fix it?
That weird thing is basically the second object you're boolean with overlapping the boolean result. When you use the boolean modifier on an object, this object becomes the result of the boolean operation. But the second object you use in that operation is still around in the scene, overlapping with the first. It's easiest to see if you turn on Random Color Shading:



Now you should be able to see the boolean result overlapped with the second one:



To fix this, you need to hide the boolean target. One way of doing this in an elegant fashion is to use Blender 2.80 collections:



Suzanne is the object that has the boolean on it. Sphere is the object I join with. I put the sphere under a new collection which I called HideMe. Now, I exclude it from the View Layer using the checkbox in front of the collection:



And as a result, the boolean object is not overlapping any longer: